1. Field of the Invention
The present invention relates to communications, and, in particular, to spread spectrum telecommunication systems such as code division multiple access (CDMA) communication systems.
2. Description of the Related Art
FIG. 1 shows the frequency spectra for a conventional frequency division multiple access (FDMA) telecommunication system comprising a base station and a plurality of mobile units (e.g., cellular telephones). The base station transmits signals to the mobile units (i.e., the forward link) using Frequency Band A, and the mobile units transmit signals to the base station (i.e., the reverse link) using Frequency Band B. In FDMA communications, each frequency band is divided into a number of different channels, where each channel corresponds to a different unique frequency sub-band. Each mobile unit is assigned a different channel in each frequency band for communications with the base station. Narrow-band interference occurs when unauthorized (e.g., jamming) signals are transmitted in at least one of the channels in at least one of the frequency bands. Although such narrow-band interference is problematic, it will adversely affect the communications of only those users who are currently communicating on those specific channels. Even if the interference cannot be stopped, at least the affected channels can be ignored (i.e., left unassigned) and communications can be supported on the other unaffected channels.
FIG. 2 shows the frequency bands or spectra for a conventional code division multiple access (CDMA) telecommunication system comprising a base station and a plurality of mobile units. In CDMA communications, each of the signals in a frequency band are spread over the entire band with different user channels being distinguished based on the assignment of different codes, such as orthogonal Walsh codes. In such a spread spectrum system, jamming or other narrow-band interference will adversely affect the communications of all of the users at the same time. In the presence of such interference, even when it is limited to a single specific frequency, a telecommunication system might be prevented from supporting any communications at all.